A foamed plastic insulated conductor such as an insulated element wire for a communication cable or an insulated core for a coaxial cable has been produced by extruding a foamed resin insulation on a conductor wire travelling through a crosshead of a gas injecting extruder. Of course, it is required that a capacitance of the foamed plastic insulated conductor is required to be generally uniform throughout its length.
In order to maintain the uniform capacitance of the foamed plastic insulated conductor, a process has been tried in which a conductor wire having a foaming agent containing insulation extruded thereon through the crosshead of the gas injecting extruder passes through cooling trough means including a trough movable axially of the insulated electric conductor toward and away from the crosshead. A capacitance monitor is provided which serves to detect a value of the capacitance of the foamed plastic insulated conductor cooled by the cooling trough means. If there is a difference between a detected value and a specified or given one of the capacitance, the movable trough is adjusted in its position to eliminate the difference in the capacitance (see Japanese Patent Application Publication No. 11,871/1973). More particularly, the capacitance of the foamed plastic insulated conductor corresponds to the expansion rate of the insulation on the conductor wire. Thus, if the detected value of the capacitance of the foamed insulated conductor is much higher than the given one of the capacitance (or if the expansion rate of the foamed insulation on the conductor wire is much lower than a given one thereof), then the movable trough is adjusted in its position so as to move away from the crosshead of the extruder. Accordingly, it takes more time to introduce the foamed plastic insulated conductor extruded out of the crosshead of the extruder, into the cooling trough means, which causes the expansion rate of the foamed insulation on the conductor wire to become higher. If the detected value of the capacitance of the foamed plastic insulated conductor is much lower than the given one thereof (or if the expansion rate of the foamed insulation is much higher), then the movable trough is adjusted in its position in a reverse manner.
The disadvantage of such a process is that the range of movement of the movable trough is limited. Thus, the capacitance of the foamed plastic insulated conductor cannot be controlled beyond the limited range of movement of the trough. In such a case, hitherto, an operator has controlled an extruder temperature or the rate of travelling the conductor wire so that the given value of the capacitance of the foamed plastic insulated conductor can be obtained. However, it disadvantageously requires a higher skill and a troublesome operation.
Another process has been proposed which is adapted to automatically control an extruder temperature in accordance with the position of the movable trough (see Japanese Patent Application Publication No. 9,837/1977). However, the heat capacity of the extruder is too large to quickly adjust the extruder temperature in accordance with the change in position of the movable trough.
Lately, there are proposed processes of producing a foamed plastic insulated conductor by extruding a resin insulation on a conductor wire while a gas as a foaming agent is being injected into a molten resin in a gas injecting extruder (see Japanese Patent Application Laying Open Gazette Nos. 53,569/1976 and 60,978/1976). In these processes, there is employed a system for controlling a capacitance of the foamed plastic insulated conductor by means of a combination of a movable trough and a capacitance monitor which are substantially identical to those in the aforementioned prior arts. In this case, since it is also possible to control the expansion rate of the foamed insulation on the conductor wire by regulating the injection rate of the foaming gas at the limit positions of the movable trough where the capacitance of the foamed plastic insulated conductor can be no longer controlled, an operator must adjust the quantity of the foaming gas injected into the extruder. However, this also disadvantageously requires a high skill of adjustment of the gas injection and a troublesome operation. In addition thereto, the adjustment of gas injection after stopping of the movable trough is not preferable because that portion of the foamed plastic insulated conductor produced after the adjustment of gas injection until the molten resin containing a predetermined amount of gas through the adjustment of gas injection reaches the outlet of the crosshead, may have defective capacitance.